System for purifying exhaust gas for use in an automobile

ABSTRACT

A system for purifying an exhaust gas for use in an automobile is disclosed which does not externally discharge unburnt hydrocarbons until a catalyst becomes active. An adsorbent is provided upstream of the catalyst. A heat exchanger is disposed between an upstream portion of the adsorbent and a portion between the adsorbent and the catalyst for controlling the elevation of the temperature of the adsorbent and for promoting the elevation of the temperature of the catalyst. The unburnt hydrocarbons are absorbed by the adsorbent in the initial period of time from starting of an engine until the catalyst becomes active. Temperature control is made in such a manner that the unburnt hydrocarbons which are adsorbed by the adsorbent begins to be desorbed therefrom substantially simultaneously with the time when the catalyst begins to function. A large quantity of unburnt hydrocarbons which are emitted from an engine immediately after the starting of the engine are prevented from being externally discharged without being treated.

This is a continuation of patent application Ser. No. 07/953,218, filedSep. 30, 1992 U.S. Pat. No. 5,388,405.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust gas purifying system for usein an automobile and in particular to an exhaust gas purifying systemfor temporarily adsorbing an automobile exhaust gas by an adsorbentuntil an exhaust gas treating catalyst becomes active.

2. Prior Art

The exhaust gas emitted from automobiles are one of the main causes ofair pollution and therefore are regulated by a number of regulations onexhaust gas emissions.

The current exhaust gas emission level regulations are complied with bythe treatment of the exhaust gas with a ternary catalyst includingplatinum, rhodium, etc. The unburnt hydrocarbons and nitrogen oxides andthe like in the exhaust gas emitted from an automobile have beenconsiderably decreased compared to emissions prior to these regulations.

However, in the prior art, the exhaust gas can not be treated with theternary catalyst until the temperature of the ternary catalyst iselevated to a temperature at which it becomes active (the light offtemperature, about 350° C.). The unburnt hydrocarbons are dischargedwithout being treated particularly immediately after starting of theengine, although the concentration of the unburnt hydrocarbons is veryhigh. If the starting temperature is, for example, 20° C., about 100seconds are taken for the catalyst to reach the light off temperature(about 350° C.). In this period of time, unburnt hydrocarbons having avery high concentration which can be as high as 7000 to 8000 ppm aredischarged.

It has been proposed as a countermeasure to this problem topreliminarily heat a catalyst with an electric heater as is known by SAEpaper 900503 and Japanese Unexamined Patent Publication No. Tokkai-Hei3-31510. These proposals have a practical difficulty since it requiressuch a large amount of electric power.

It has been known that unburnt hydrocarbons are adsorbed by an adsorbentupstream of a catalyst until the catalyst is activated (JapaneseUnexamined Patent Publication No. Tokkai-Sho 63-68713).

Recently, air pollution has become a more serious concern again, sincethe absolute amount of the emitted exhaust gas has increased due to theincrease in the number and the size of automobiles. It has been decidedthat stricter exhaust gas regulations will be enforced as a solution forthis air pollution problem. For example, it has been decided that LEV(Low Emission Vehicle) regulation will be enforced in California, U.S.A.from 1997. This regulation will be enforced in all States in the U.S.A.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an exhaustgas purifying system for use in an automobile which is capable oftreating an emitted exhaust gas immediately after starting of an engineuntil a catalyst is heated up to a temperature at which the catalystbecomes active.

In an aspect of the present invention, there is provided a process forpurifying an exhaust gas for use in an automobile in which an exhaustgas treating catalyst and an adsorbent having a temperature range atwhich the adsorbent adsorbs unburnt hydrocarbons (hereinafter referredto as "adsorption zone") and a temperature range at which the adsorbentdesorbs the adsorbed unburnt hydrocarbons (hereinafter referred to as"desorption zone") are disposed in an exhaust gas passage from an engineand the unburnt hydrocarbons in the exhaust gas are temporarily adsorbedby the adsorbent when the catalyst does not function, wherein thetemperature control of at least one of the adsorbent and catalyst iscarried out in such a manner that the time when the temperature of theadsorbent reaches the desorption zone coincides with the time when thetemperature of the catalyst reaches a temperature at which the catalystbecomes active.

In another aspect of the present invention, there is provided a systemfor purifying an exhaust gas for use in an automobile which comprises anexhaust gas treating catalyst disposed in an exhaust gas passage from anengine, an adsorbent having a temperature range at which the adsorbentadsorbs unburnt hydrocarbons and a temperature range at which theadsorbent desorbs the adsorbed unburnt hydrocarbons, which is disposedin the exhaust gas passage upstream of the catalyst and temperaturecontrol means for maintaining the temperature of the adsorbent so thatunburnt hydrocarbons contained in the exhaust gas are temporarilyadsorbed by the adsorbent when the catalyst does not function and forbringing the temperature of the adsorbent into the desorption zone todesorb the adsorbed unburnt hydrocarbons when the catalyst begins tofunction.

The temperature control means comprises a heat exchanger which isdisposed between a portion of the exhaust gas passage upstream of theadsorbent and a portion of the exhaust gas passage between the adsorbentand the catalyst.

Air supplying means for supplying air to the exhaust gas passage isprovided in the exhaust gas passage upstream of the catalyst.

The system for purifying an exhaust gas for use in an automobile mayfurther include means for detecting the amount of air which is suppliedto the engine, means for adjusting the pressure in the adsorbent in theexhaust gas passage and means for controlling the pressure controllingmeans based upon the detection result of the air amount detecting means.

The system for purifying an exhaust gas for use in an automobile mayfurther include means for detecting the pressure of the exhaust gas inthe adsorbent, means for adjusting the pressure in the adsorbentdisposed in the exhaust gas passage and means for controlling thepressure controlling means based upon the detection result of the airamount detecting means.

In a further aspect of the present invention, there is provided a systemfor purifying an exhaust gas for use in an automobile which comprises anexhaust gas main passage for discharging the exhaust gas from an engine,a bypass passage for connecting an upstream portion of the exhaust gasmain passage with a downstream portion of the exhaust gas main passage,adsorbent disposed in the bypass passage having a temperature range atwhich the adsorbent adsorbs unburnt hydrocarbons, a temperature rangeabove the adsorption zone, at which the adsorbent desorbs the adsorbedunburnt hydrocarbons therefrom and a further higher temperature range atwhich the adsorbed material which can not be completely desorbed in thedesorption zone can be eliminated from the adsorbent (hereinafterreferred to as regeneration zone), exhaust gas treating catalystprovided in the exhaust gas passage in parallel with the bypass passage,means for detecting the temperature of the adsorbing means and/or thecatalyst, air supplying means disposed in the bypass passage downstreamof the adsorbent, passage changing means provided at the exit of thebypass passage for adjusting the ratio of the flow rate of the exhaustgas through the exhaust gas main passage to that through the bypasspassage, and means for controlling the passage changing means in such amanner that the exhaust gas is caused to flow through the bypass passagewhen the temperature of the adsorbent falls in the adsorption zone andthe bypass passage is closed to cause the exhaust gas to flow throughthe main passage and air is supplied to the bypass passage by the airsupply means when the temperature of the adsorbent falls in thedesorption zone.

The system for purifying an exhaust gas for use in an automobile mayfurther include subsidiary bypass means for introducing the exhaust gasinto the bypass passage in the downstream side of the adsorbent from theexhaust gas main passage and the control means being capable ofelevating the temperature of the adsorbent up to the regeneration zoneby causing the exhaust gas to flow into the bypass passage via thesubsidiary bypass passage.

In a further aspect of the present invention, there is provided a systemfor purifying an exhaust gas for use in an automobile which comprises anexhaust gas main passage for discharging the exhaust gas from an engine,a bypass passage for connecting an upstream portion of the exhaust gasmain passage with a downstream portion of the exhaust gas main passage,adsorbent disposed in the bypass passage having a temperature range atwhich the adsorbent adsorbs unburnt hydrocarbons, a temperature rangeabove the adsorption zone, at which the adsorbent desorbs the adsorbedunburnt hydrocarbons therefrom and a further higher temperature range atwhich the adsorbed materials which can not be completely desorbed in thedesorption zone can be eliminated therefrom, exhaust gas treatingcatalyst provided in the exhaust gas passage in parallel with the bypasspassage, means for detecting the temperature of the adsorbing meanand/or the catalyst, air supplying means disposed in the bypass passageupstream of the adsorbent, passage changing means provided at the exitof the bypass passage for adjusting the ratio of the flow rate of theexhaust gas through the exhaust gas main passage to that through thebypass passage, exhaust gas recirculating means which connects thebypass passage downstream of the adsorbent with an air intake system ofthe engine for returning a desired amount of the exhaust gas to the airintake system, means for controlling the passage changing means in sucha manner that the exhaust gas is caused to flow through the bypasspassage when the temperature of the adsorbent falls in the adsorptionzone and the bypass passage is closed to cause the exhaust gas to flowthrough the main passage and air is supplied to the bypass passage bythe air supply means when the temperature of the adsorbent falls in thedesorption zone.

The system for purifying an exhaust gas for use in an automobile mayfurther include subsidiary bypass means for introducing the exhaust gasinto the bypass passage in the upstream side of the adsorbent from theexhaust gas main passage. The control means is preferably capable ofelevating the temperature of the adsorbent up to the regeneration zoneby causing the exhaust gas to flow into the bypass passage via thesubsidiary bypass passage.

The system may further include means for detecting the amount of airsupplied to the engine and the control means is preferably capable ofcontrolling the amount of the exhaust gas which is recirculated by theexhaust gas recirculating means based upon the detection result of theair amount detecting means.

In a further aspect of the present invention, there is provided a systemfor purifying an exhaust gas for use in an automobile which comprises anexhaust gas main passage for discharging the exhaust gas from an engine,a bypass passage for connecting an upstream portion of the exhaust gasmain passage with a downstream portion of the exhaust gas main passage,adsorbent disposed in the bypass passage having a temperature range atwhich the adsorbent adsorbs unburnt hydrocarbons, a temperature rangeabove the adsorption zone, at which the adsorbent desorbs the adsorbedunburnt hydrocarbons therefrom and a further higher temperature range atwhich the adsorbed material which can not be completely desorbed in thedesorption zone can be eliminated therefrom, exhaust gas treatingcatalyst provided in the exhaust gas passage downstream of the bypasspassage, means for detecting the temperature of the adsorbing meanand/or the catalyst, air supplying means disposed in the exhaust passageupstream of the bypass passage, passage changing means provided at theentrance of the bypass passage for adjusting the ratio of the flow rateof the exhaust gas through the exhaust gas main passage to that throughthe bypass passage, and means for controlling the passage changing meansin such a manner that the exhaust gas is caused to flow through thebypass passage when the temperature of the adsorbent falls in theadsorption zone and the bypass passage is closed to cause the exhaustgas to flow through the main passage and air is supplied to the bypasspassage by the air supply means when the temperature of the adsorbentfalls in the desorption zone and the catalyst does not function and theexhaust gas is caused to flow to the bypass passage when the catalystfunctions.

A second catalyst having an activation temperature lower than that ofthe catalyst is preferably provided in the bypass passage.

The temperature of the adsorbent is maintained in the adsorption zone sothat the unburnt hydrocarbons are temporarily adsorbed by the adsorbentuntil the temperature of the catalyst is elevated so that the catalystbecomes active. Temperature control is conducted in such a manner thatthe adsorbent reaches the desorption temperature simultaneously with thetime when the catalyst reaches the light off temperature. This willcause the unburnt hydrocarbons which were temporarily adsorbed by theadsorbent to be treated with the catalyst.

This temperature control is automatically achieved if the light offtemperature of the catalyst matches with the desorption zone of theadsorbent. If they do not match with each other, the temperature controlis enabled by eliminating heat from the exhaust gas in a positionupstream of the adsorbent by means of, for example, a heat exchanger sothat the heat is used for heating the catalyst.

In the above mentioned operation, the adsorption efficiency can beenhanced by controlling the pressure in the adsorber depending upon theamount of air supplied to the engine which is detected by the air amountdetecting means and the pressure in the adsorber which is detected bythe pressure detecting means.

Deviation of the air/fuel ratio of the exhaust gas from thestoichiometric air/fuel ratio due to the presence of the desorbedunburnt hydrocarbons is prevented by supplying air upstream of thecatalyst from the air supplying means.

Initially, the exhaust gas is caused to flow through the bypass passageby the passage changing means so that the unburnt hydrocarbons areadsorbed. When the temperature of the adsorbent reaches the desorptionzone, the exhaust gas is caused to flow through the main passage bymeans of the passage changing means. On the other hand, air is suppliedto the bypass passage by means of the air supplying means so that thedesorbed unburnt hydrocarbons flows through the bypass passage in areverse direction and is returned to the main passage or returned to theair intake system of the engine by means of the exhaust gasrecirculating means. In this case, the amount of air supplied by the airsupplying means is determined based upon the detection result of the airamount detecting means.

In the case where the catalyst is disposed in parallel with the bypasspassage, the catalyst is heated on adsorption by causing the exhaust gasto flow through the subsidiary bypass passage. On the other hand, theadsorbent is heated on desorption and regeneration of the adsorbent.

An arrangement in which the catalyst is disposed downstream of thebypass passage will be described.

When the temperature of the adsorbent is in the desorption zone and thetemperature of the catalyst does not reach the light off temperature,the exhaust gas is caused to flow to the main passage while air issupplied to the exhaust gas main passage by the air supplying meanswhich is provided upstream of the catalyst. This enables the unburnthydrocarbons to be burnt in the main passage.

In accordance with the present invention, the unburnt hydrocarbons canbe adsorbed even if the catalyst has not been sufficiently warmed upimmediately after the starting of the engine.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing the characteristics of anadsorbent;

FIG. 2 is a schematic diagram showing a first embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 3 is a schematic diagram showing a second embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 4 is a schematic diagram showing a third embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 5 is a schematic diagram showing a fourth embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 6 is a schematic diagram showing a fifth embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 7 is a schematic diagram showing a sixth embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 8 is a schematic diagram showing a seventh embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 9 is a schematic diagram showing an eighth embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 10 is a schematic diagram showing a ninth embodiment of an exhaustgas purifying system of the present invention for use in an automobile;

FIG. 11 is a graph showing a rule for temperature control; and

FIG. 12 is an explanatory view showing the inputs and outputs to andfrom a control unit in an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments of the present invention will be described withreference to the Figures.

A basic concept of the present invention resides in that unburnthydrocarbons are temporarily adsorbed by an adsorbent and are notdischarged until a catalyst reaches an activity exhibiting temperature,in other words, when treatment of an exhaust gas with the catalystbecomes possible. The adsorbed unburnt hydrocarbons are desorbed andreleased for the final treatment with the catalyst when the catalystbecomes active.

An adsorbent which is essential in the present invention will firstly bedescribed.

An adsorbent having a property to adsorb an desired material, unburnthydrocarbons in the present embodiment, should of course be used. Itsproperty depends upon the temperature.

The adsorbent adsorbs unburnt hydrocarbons at low temperature as shownin FIG. 1. This temperature zone is represented as an adsorption zone AZin the drawing. The adsorbed unburnt hydrocarbons are desorbed at highertemperatures. This temperature zone is represented as a desorption zoneDZ. The adsorbed unburnt hydrocarbons are not completely desorbed whenthe temperature reaches the desorption temperature. Some of the adsorbedunburnt hydrocarbons remain adsorbed. In order to remove such unburnedhydrocarbons, they are burnt, by elevating the temperature of theadsorbent even further. This zone is represented as a regeneration zoneRZ.

It is necessary that a temperature limit (represented as thermalresistance zone TRZ) at which the catalyst, etc. are broken be at aneven higher temperature range.

The present invention utilizes the temperature characteristic of such anadsorbent. In other words, the temperature of the adsorbent ismaintained in the adsorbing zone so that the adsorbent adsorbs unburnthydrocarbons until the catalyst is activated. After the activation ofthe catalyst, the temperature of the adsorbent is elevated to thedesorbing zone for desorbing and releasing the adsorbed unburnthydrocarbons. If much undesorbed unburnt hydrocarbons remains so thatthe adsorbing capacity of adsorbent is lowered, the adsorbent isregenerated by elevating the temperature to the regeneration zone toincrease the adsorbing capacity again.

Such an adsorbing system per se is completely independent of the exhaustgas treatment with a catalyst. It is thus necessary to independentlyperform the temperature control of the adsorbing system. Thetemperatures of the adsorbing zone, etc. depend upon the kind ofcatalyst.

Accordingly, it is not necessary to independently perform thetemperature control of the adsorbing system if the characteristics ofthe adsorbent match with those of the catalyst. It suffices to perform aconsolidated temperature control for both the adsorbent and thecatalyst. If they do not properly match with each other, it is necessaryto perform the temperature control of the adsorbing system independentlyof that of the catalyst. In other words, it is necessary tocorrespondingly perform the temperature control of the adsorbing systemand the catalyst system by changing, for example, the structure ofpipings, etc. depending upon the temperature characteristics of theadsorbent and the catalyst. This temperature control will be describedhereafter in the following description of the embodiments.

The characteristics of the adsorption have been described with referenceto only the influence by temperature. They may however, be influenced bypressure, space velocity, etc. as well as temperature. The adsorptionrate becomes higher as the pressure becomes higher while conversely thedesorption rate becomes faster as the pressure becomes lower. As thespace velocity becomes lower, the adsorption rate becomes higher.

Various embodiments of an exhaust gas purifying system for use in anautomobile will be described hereafter.

A first embodiment in which an adsorbent is disposed upstream of acatalyst in an exhaust gas passage is shown in FIG. 2.

In the present embodiment, an adsorber 4a which contains an adsorbent 4therein is provided in an exhaust gas passage which is connected with anengine 1. A main catalytic converter 8a containing a main catalyst 8 isdisposed in the passage downstream of the adsorber 4a.

A secondary air pump 9 and a secondary air amount control valve 10 areprovided in the passage upstream of the adsorber 4a. A pressure controlvalve 6 is provided between the adsorber 4a and the main catalyticconverter 8a.

The adsorbent having the above mentioned desorption zone in atemperature range which is not lower than the light off temperature ofthe main catalyst 8 is most preferable in the present embodiment. Inother words, if the temperature characteristics of the adsorbent 4 matchthose of the main catalyst 8 so that it is not necessary toindependently control only the temperature of the adsorbent 4, thepresent invention may take a structure like the present embodiment.Specifically, it is preferable that the temperature ranges of thedesorption and regeneration zones fall into a temperature range attainedby the exhaust gas after an automobile having the engine 1 has beenrunning for a while. In this case, desorption and regeneration of theadsorbent will be conducted by the usual running of the automobile. Ifthe desorption characteristics are excellent, no regeneration zone needexist. Since the exhaust gas passes through the adsorber 4a withoutbeing adsorbed in the usual running, the adsorbent 4 should have enoughheat resistance which is equal to or higher than that of the maincatalyst. (The adsorbent 4 may be heated to a temperature which ishigher than the temperature of the main catalyst 8 since the adsorbent 4is provided upstream of the main catalyst 8.)

It is of course necessary to adsorb the unburnt hydrocarbons in thetemperature range on starting of the engine.

The adsorber 4a is provided with an exhaust gas temperature sensor 2, anadsorbent temperature sensor 3 and an exhaust gas temperature sensor 5for accurately measuring the temperature. The method of measuring thetemperature by these sensors will be described hereafter with referenceto the other structures.

The main catalyst 8 is made of a ternary catalyst. The main catalyticconverter 8a is provided with a catalyst temperature sensor 7 formeasuring the temperature of the main catalyst 8. The temperature whichis measured by the catalyst temperature sensor 7 is used for controllingvarious valves. The method of controlling the valves is not describedherein in detail.

The secondary air pump 9 and the secondary air amount control valve 10are provided to maintain the concentration of oxygen of the exhaust gasflowing into the main catalyst 8 at a value suitable for treatment. Evenif the air/fuel mixture supplied to the engine 1 has the stoichiometricair/fuel ratio, due to the presence of the unburnt hydrocarbons desorbedfrom the adsorbent 4, the concentration of the oxygen of the exhaust gasreaching the main catalyst 8 may be low. In this occasion, there is ashortage of oxygen for treatment of all the unburnt hydrocarbons.

The pressure control valve 6 is provided for adjusting the spacevelocity, i.e. the flow rate of the exhaust gas. This decreases thespace velocity of the exhaust gas into a range in which the conditionsof the temperature, etc. allow the adsorpability of the adsorbent forthe unburnt hydrocarbons to be enhanced. Data on the amount of theexhaust gas is necessary to control the pressure control valve 6. Theamount of air which is taken in through an air cleaner 17 is measured byan air flow rate detector 16 for calculating the amount of the exhaustgas based upon the measured amount of the intake air. In thisembodiment, the amount of the intake air approximates to the amount ofthe exhaust gas. Although not described in detail, the amount of theintake air which is measured by the air amount detector 16 is not onlyused for controlling the pressure control valve 6, but is also used forcontrolling the various valves in the alternative embodiments which willbe described hereafter.

Adjusting the space velocity of the exhaust gas will adjust the pressureof the exhaust gas in the adsorber 4a. In this case, the temperature ofthe exhaust gas may change however, this change in temperature isnegligibly small. The exhaust gas purifying system for use in anautomobile therefore capable of controlling the space velocity of theexhaust gas substantially independently of the temperature.

The operation in the first embodiment will now be described.

The main catalyst 8 does not function when the engine 1 is started sincethe temperature of the catalyst 8 is low. The unburnt hydrocarbons arenot discharged since the adsorbent 4 adsorbs the unburnt hydrocarbons inthe exhaust gas even at the low temperatures.

When the temperature of the main catalyst 8 is elevated to the light offtemperature, the main catalyst 8 will begin to treat the unburnthydrocarbons, etc. Substantially simultaneously with this, thetemperature of the adsorbent 4 rises into the desorption zone anddesorbs the adsorbed unburnt hydrocarbons. Under this condition, theexhaust gas emitted from the engine 1 and the unburnt hydrocarbonsdesorbed from the adsorbent 4 are treated with the main catalyst 8. Inthis case, at the catalytic converter 8a it is possible to run short ofoxygen. This phenomenon can be adjusted by supplying additional air tothe catalytic converter 8a from the secondary air pump 9. An oxygendetecting means may be provided immediately upstream of the catalyticconverter 8a for controlling the secondary air pump 9, etc. based uponthe result of detection. This enables accurate control of theconcentration of the oxygen in the exhaust gas.

Regeneration of the adsorbent 4 will be automatically achieved if thetemperature of the adsorbent 4 is elevated by the hot exhaust gas afterrunning of the vehicle for a while.

The above mentioned operation is performed in accordance withinstructions from a control unit 23 shown in FIG. 12. The control unit23 will be described later.

A second embodiment in which the system of the first embodiment isprovided with a preliminary catalytic converter 11a containing apreliminary catalyst 11 and a pressure detector 12 is shown in FIG. 3.

In this embodiment, providing of the pressure detector 12 enables apressure control valve 6 to be accurately controlled so that thecapacity of the adsorbent can be utilized at its maximum.

The preliminary catalytic converter 11a which is provided upstream of anadsorber 4a, i.e. in a position close to the exhaust slot of an engine1, for example, is directly mounted on an exhaust manifold to enableshortening of the period of time which is taken to obtain the activityof the catalyst since the elevation in the temperature of thepreliminary catalyst 11 is fast. Although the preliminary catalyst 11may be identical with the main catalyst 8, it is necessary to use acatalyst having a high thermal resistance since it is disposed in aposition close to the engine 1.

A third embodiment will be described with reference to FIG. 4.

In this embodiment, a bypass exhaust pipe 13 is provided in the exhaustgas passage and the adsorber 4a containing the adsorbent 4 is disposedin the bypass exhaust pipe 13. A secondary air pump 9 and a secondaryair amount control valve 10 for adjusting the air/fuel ratio (oxygenconcentration) of the exhaust gas are connected to the bypass exhaustpipe 13 downstream of the adsorber 4a containing the adsorbent 4. Anexhaust gas passage change-over valve 18 for controlling the admissionof the exhaust gas into the bypass exhaust pipe 13 is provided at theexit of the bypass exhaust pipe 13. Switching between the main passage,provided with the main catalytic converter 8a and the bypass exhaustpipe 13 through which the exhaust gas emitted from the engine 1 flowsthrough can be made by actuating the change-over valve 18. The openingof the exhaust passage change-over valve 18 is adjustable so that someof the exhaust gas can flow through the bypass exhaust pipe 13.

The reason why the secondary air pump 9 and the secondary air amountcontrol valve 10 are disposed downstream of the adsorber 4a and theexhaust passage change-over valve 18 is disposed at the exit of thebypass exhaust pipe 13 is that a backflow of the exhaust gas through thebypass exhaust pipe 13 may otherwise occur when the unburnt hydrocarbonsare desorbed from the adsorbent 4.

The main catalytic converter 8a is disposed in the main passage of theexhaust gas passage in parallel with the adsorber 4a. Since the exhaustgas does not flow through the main catalytic converter 8a while theexhaust gas flows through the bypass exhaust pipe 13, the elevation intemperature of the main catalyst 8 is slow. The slowness of theelevation in temperature of the main catalyst 8 can be sufficientlycompensated for by modifying the geometrical configuration of piping.This may be achieved, for example, by shortening the distance betweenthe main catalytic converter 8a and a branch between the bypass exhaustpipe 13 and the main passage or by arranging the bypass exhaust pipe 13around the main catalytic converter 8a in a spiral manner. The reasonwhy the exhaust passage change-over valve 18 is disposed downstream ofthe bypass exhaust pipe 13 is that even when the exhaust gas is causedto flow through the bypass exhaust pipe 13, the passage per se is notinterrupted at the entrance of the bypass exhaust pipe 13 so that theheat in the exhaust gas is ready to conduct to the main catalyst 8 inthe converter 8a.

An A/F (air/fuel ratio) sensor 21 is provided between a main catalyticconverter 8a and the branch between the main flow line and the bypassexhaust pipe 13 for monitoring the change in the concentration of theoxygen of the exhaust gas flowing into the main catalytic converter 8awhich is caused by the unburnt hydrocarbons desorbed from the adsorbent4. The detection result from the A/F sensor 21 is used to control theabove mentioned secondary air pump 9 and the secondary air amountcontrol valve 10 so that a suitable concentration of oxygen for thetreatment by the main catalyst 8 is maintained.

Since the arrangement of the adsorbent 4 in the bypass exhaust pipe 13enables the control of the adsorbent temperature to be performedindependently of that of the catalyst in the present embodiment, whetheror not the desorption zone matches with the characteristics of the maincatalyst 8 will not matter. This similarly applies to the regenerationzone and the heat resistance zone. However, it is necessary to performthe control of the adsorbent temperature to provide such an adsorptionzone that the unburnt hydrocarbons are adsorbed also in the temperaturerange on starting of the engine 1, similarly to the foregoingembodiments.

An exhaust gas temperature sensor 2, adsorbent temperature sensor 3,exhaust gas temperature sensor 5 and catalyst temperature sensor 7 areidentical with corresponding sensors in the foregoing embodiments.Detection of the temperature will be described in detail hereafter. Thetemperature control means comprises the bypass exhaust pipe 13, theexhaust passage change-over valve 18, the temperature sensors 2, 3, 5, 7and a control unit 23 (shown in FIG. 12) for controlling the exhaustpassage change-over valve 18 in response to the output from each of thetemperature sensors.

When the engine 1 is started, the exhaust passage change-over valve 18is in such a position a, as represented by a dotted line, that theunburnt hydrocarbons in the exhaust gas are adsorbed by the adsorbent 4and are not externally discharged.

The temperatures of the adsorbent 4 and the main catalyst 8 aremonitored based upon the outputs from the exhaust gas temperature sensor2, the adsorbent temperature sensor 3, the exhaust gas temperaturesensor 5 and the catalyst temperature sensor 7. When the temperature ofthe adsorbent 4 comes close to the desorption zone, the exhaust passagechange-over valve 18 is gradually opened so that a part of the exhaustgas passes through the main catalytic converter 8a. The main catalyst 8in the converter has already been warmed to some extent and thetemperature of the exhaust gas per se is higher than that immediatelyafter the starting of the engine. Accordingly, the main catalyst 8quickly reaches the light off temperature.

When it is confirmed by the catalyst temperature sensor 7 that thetemperature of the main catalyst 8 has been sufficiently elevated, theexhaust passage change-over valve 18 is brought into a position b sothat all the exhaust gas passes through the main catalytic converter 8a.Thereafter, the exhaust gas newly emitted from the engine 1 will thus bedirectly treated with the main catalyst 8.

When the exhaust passage change-over valve 18 is brought into theposition b, the temperature of the adsorbent 4 is in the desorptionzone.

Accordingly, by operating the secondary air pump 9 and the secondary airamount control valve 10 in the position b of the valve 19 the unburnthydrocarbons which are desorbed from the adsorbent 4 are caused to flowthrough the bypass exhaust pipe 13 in a reverse direction and then topass through the main catalytic converter 8a so that they are treatedwith the main catalyst 8.

At this time, the concentration of the oxygen of the exhaust gas ismonitored by the A/F sensor 21 and the amount of air fed to the bypassexhaust pipe 13 is adjusted by the secondary air pump 9 and thesecondary air amount control valve 10 in order not to be short of oxygenin the exhaust gas passed through the main catalytic converter 8a, dueto the introduction of the desorbed unburnt hydrocarbons.

Elevation in temperature of the adsorbent 4 which is required for theregeneration thereof can be sufficiently obtained from the heatconducted from the exhaust gas passing through the main catalyticconverter 8a if the layout and the geometrical configuration of thebypass exhaust pipe 13 is modified as is similar to the heating of themain catalyst 8 as mentioned above.

When the exhaust gas is caused to flow through the bypass exhaust pipe13 in a reverse direction, unburnt hydrocarbons are externallydischarged without being treated until the main catalyst 8 reaches thelight off temperature. This is not a serious problem since a certainperiod of time has lapsed from the starting of the engine and the levelof unburnt hydrocarbons in the exhaust gas is less than that immediatelyafter the engine is started and this period of time is short.Accordingly, the desorption temperature of the adsorbent 4 necessarilyneed not be equal to the light off temperature of the main catalyst 8.This problem can be easily solved if the temperature of the adsorbent 4is controlled by separate means for introducing air into the bypassexhaust pipe 13 in a position upstream of the adsorbent 4 so that thetemperature of the adsorbent 4 does not reach the desorption zone untilthe main catalyst 8 reaches the light off temperature.

The above mentioned operation is all performed in accordance withinstructions from the control unit 23 shown in FIG. 12. The control unit23 will be described in the last part of the specification.

A fourth embodiment will be described with reference to FIG. 5.

The present embodiment is substantially identical with the thirdembodiment shown in FIG. 4 except that the desorbed unburnt hydrocarbonsdo not flow back through the bypass exhaust pipe 13 for passing throughthe main catalytic converter 8a, but are returned to an intake system ofthe engine 1 through an EGR (Exhaust Gas Recirculation) valve 15 whichis provided downstream of the adsorber 4a of the bypass exhaust pipe 13.The secondary air pump 9 and the secondary air amount control valve 10are disposed upstream of the adsorber 4a.

The operation in the present embodiment is substantially identical withthat in the foregoing embodiments. The EGR valve 15 is controlled sothat it is opened when the adsorbed unburnt hydrocarbons are desorbed,i.e. when the exhaust passage change-over valve 18 is in the position b.This control is made depending upon the intake air amount which ismeasured by the air amount detector 16.

A heat source which is necessary for the regeneration of the adsorbercan be provided by modifying the layout of the exhaust flow line.Alternatively, the adsorbent 4 may be heated by the introduction of theexhaust gas into the bypass exhaust pipe 13 by more or less opening theexhaust passage change-over valve 18 or the EGR valve 15.

In this embodiment, the exhaust gas does not flow through the bypassexhaust pipe 13 in a reverse direction so that smooth control can bemade.

A fifth embodiment will be described with reference to FIG. 6.

The present embodiment is substantially identical with the embodiment ofFIG. 5 except that the exhaust gas is introduced to the bypass exhaustpipe 13 from the main passage of the exhaust gas passage as a heatsource for heating the adsorbent 4 to the regeneration zone for moresmoothly conducting the regeneration of the adsorbent 4.

In this embodiment, an exhaust gas introducing valve 25 is provideddownstream of the catalytic converter 8a in the main exhaust gas passagewhich is parallel with the bypass exhaust pipe 13. A subsidiary bypassexhaust pipe 22 is provided for introducing the exhaust gas flowing viathe exhaust gas introducing valve 25 into the bypass exhaust pipe 13upstream of the adsorber 4a.

The opening of the exhaust gas introducing valve 25 is adjusted based onthe results detected by the exhaust gas temperature sensor 2, theexhaust gas temperature sensor 5, and the catalyst temperature sensor 7.

The operation of the present embodiment will be described.

The operation of the present embodiment is substantially identical withthat of the embodiment of FIG. 5.

When the unburnt hydrocarbons are adsorbed in the adsorbent 4immediately after the start of the engine 1, the exhaust gas passagechange-over valve 18 is brought into a position a so that exhaust gaspasses through the bypass exhaust pipe 13. The exhaust gas is caused toflow through the main catalytic converter 8a to some extent by slightlyopening the exhaust gas introducing valve 25 in this position a of thevalve 18. This causes the temperature of the main catalyst 8 to bequickly elevated.

Regeneration of the adsorbent 4 is conducted during the usual running ofthe vehicle, i.e. while the temperature of the main catalyst 8 has beensufficiently elevated and the treatment of the exhaust gas is performedwith the main catalyst 8.

On regeneration of the adsorbent, the exhaust gas introducing valve 25is slightly opened to cause a part of the hot exhaust gas which haspassed through the main catalyst 8 to flow to the upstream side of theadsorber 4a. This enables the adsorbent 4 to be heated to a sufficientlyhigh temperature. At this time, the opening of the exhaust gasintroducing valve 25 is adjusted based upon the temperatures which aremeasured by the exhaust gas temperature sensor 2, the adsorbenttemperature sensor 3, the exhaust gas temperature sensor 5 and thecatalyst temperature sensor 7 so that the optimum temperature for theregeneration of the adsorbent can be attained.

In the present embodiment, the heat necessary for the regeneration ofthe adsorbent can be positively provided and the flexibility in choiceof the geometrical configuration and the layout of the bypass exhaustpipe 13, etc. is enhanced. Since the heat necessary for the regenerationis obtained from the exhaust gas containing no unburnt hydrocarbons,etc. which has been treated with the main catalyst 8, regeneration ofthe adsorbent can be completely conducted.

A sixth embodiment will be described with reference to FIG. 7.

The present embodiment shown in FIG. 7 is substantially identical withFIG. 4 except that an exhaust gas introducing valve 27 is provideddownstream of the main catalytic converter 8a so that the exhaust gasintroduced via the exhaust gas introducing valve 27 is introduced to thedownstream side of the adsorber 4a in the bypass exhaust pipe 13 througha subsidiary bypass exhaust pipe 29. The heat necessary for theregeneration of the adsorber 4 is obtained from the exhaust gasintroduced through the subsidiary bypass exhaust pipe 29. The opening ofthe exhaust gas introducing valve 27 is adjustable based upon theresults detected by the exhaust gas temperature sensor 2 and theadsorbent temperature sensor 3, etc. Although the subsidiary bypassexhaust pipe 29 is in communication with the bypass exhaust pipe 13downstream of the position where the secondary air amount control valve10 is provided in the bypass exhaust pipe 13, the subsidiary exhaustpipe 13 may be in communication with the bypass exhaust pipe 29 betweenthe adsorber 4a and the secondary air amount control valve 10.

The operation of the sixth embodiment will be described.

The basic operation of the present embodiment is substantially identicalwith that of the embodiment shown in FIG. 4.

When the adsorbent 4 is regenerated, the exhaust gas introducing valve27 is opened to cause the hot exhaust gas to pass through the adsorbent4 so that the adsorbent 4 can be easily heated up to the regenerationzone. The opening of the exhaust gas introducing valve 27 is adjustedbased upon the detection results from the exhaust gas temperature sensor2, the adsorbent temperature sensor 3, the exhaust gas temperaturesensor 5 and the catalyst temperature sensor 7 so that the optimumtemperature can be attained.

In the present embodiment, the heat necessary for the regeneration ofthe adsorbent can be easily obtained. The restriction of the geometricalconfiguration, the layout, etc. of the bypass exhaust pipe 13 to obtainthe heat necessary for the regeneration is reduced so that flexibilityin design is increased. It is also possible to conduct sufficientdesorption of the unburnt hydrocarbons by introducing the exhaust gas tothe adsorber 4a via the subsidiary exhaust pipe 29.

Now, a seventh embodiment will be described with reference to FIG. 8.

In the present embodiment, only one exhaust passage is provided and nobypass is provided as is similar to the first embodiment shown in FIG.2. The adsorber 4a and the main catalytic converter 8a are seriallyarranged in the exhaust passage as is similar to the first embodiment.However, the present embodiment is different from the first embodimentof FIG. 2 in that a heat exchanger 19 is provided upstream of theadsorber 4a.

The exhaust gas passage is arranged in such a manner that heat isexchanged between a portion of the line upstream of the adsorber 4a anda portion of the line between the adsorber 4a and the main catalyticconverter 8a via the heat exchanger 19. This arrangement makes itpossible to independently adjust the temperature of the adsorbent 4 tosome extent without preventing the elevation in temperature of the maincatalyst 8 although the adsorber 4a and the main catalytic converter 8aare serially arranged. The desorption zone of the adsorbent 4 used inthe present embodiment may be lower than the light off temperature ofthe main catalyst 8 as long as it is in a range which is adjustable bythe heat exchanger 19. The thermal resistance of the adsorbent 4 may belower than that in the case where there is no heat exchanger 19.

The operation of the present embodiment will be described.

The exhaust gas which is emitted from the engine immediately afterstarting of the engine 1 loses some of its heat through the heatexchanger 19 resulting in the temperature of the exhaust gas beinglowered to some extent. Accordingly, a large quantity of heat is notdirectly given to the adsorbent 4. However, the unburnt hydrocarbons inthe exhaust gas are not influenced by the presence of the heat exchanger19 and are passed through the heat exchanger 19 and then adsorbed by theadsorbent 4.

On the other hand, since the heat which has been removed by the heatexchanger 19 is returned to the exhaust gas flow line in the upstreamside of the main catalytic converter 8 again, the period of time whichis taken for the main catalyst 8 to reach the light off temperature isshortened correspondingly to the fact that the temperature of theadsorbent 4 is not elevated. Conversely, it is possible to delay thetime when the temperature of the adsorbent 4 reaches the desorption zoneuntil the main catalyst 8 reaches the light off temperature.

The temperature of the adsorbent 4 can be easily elevated to reach theregeneration zone by adjusting the capability of the heat exchanger 19when the adsorbent 4 is to be regenerated.

Although not described in detail, the adjustment of the capability ofthe heat exchanger 19 is performed based upon the results of detectionfrom the exhaust gas temperature sensor 2, the adsorbent temperaturesensor 3, the exhaust gas temperature sensor 5 and the catalysttemperature sensor 7, etc. This makes it possible to perform accuratecontrol of the temperature. Although the heat exchanger 19 is providedseparately from the main catalytic converter 8, they may be integrallyprovided.

Now, an eighth embodiment will be described with reference to FIG. 9.

The present embodiment has a feature that the unburnt hydrocarbons areprevented from being externally discharged by combusting the unburnthydrocarbons in the exhaust gas passage in the transition from theadsorbent to the catalyst.

The exhaust gas flow line is provided with the bypass exhaust pipe 13,and the adsorber 4a is disposed in the bypass exhaust pipe 13. The maincatalytic converter 8a is disposed in the main flow line downstream ofthe connection between the main flow line and the bypass exhaust pipe 13so that all the exhaust gas always passes through the main catalyticconverter 8a.

An exhaust passage change-over valve 31, the opening of which isadjustable is provided at the entrance of the bypass exhaust pipe 13. Aflow rate control valve 14 which controls the flow rate of the exhaustgas flowing through the bypass exhaust pipe 13 is provided at the exitof the bypass exhaust pipe 13. Selection whether the exhaust gas reachesthe main catalytic converter 8a via the adsorber 4a or directly reachesthe main catalytic converter 8a without passing through the adsorber 4acan be made by controlling these valve 31 and 14. The adsorbent 4 whichis used in the present embodiment may have the desorption zone which islower than the light off temperature of the catalyst 8. The thermalresistance of the adsorbent 4 need not necessarily be as high as that ofthe catalyst.

The secondary air pump 9 and the secondary air amount control valve 10for burning the unburnt hydrocarbons and for adjusting the concentrationof the oxygen are provided upstream of the branch between the mainpassage of the exhaust gas passage and the bypass exhaust pipe 13.

The exhaust temperature sensor 2, the adsorbent temperature sensor 3,the exhaust gas temperature sensor 5 and the catalyst temperature sensor7 are identically with those in the foregoing embodiment. The details ofthese sensors will be described hereafter.

On starting of the engine, the exhaust passage change-over valve 31 isbrought into a position a, i.e. a completely closed position to causethe exhaust gas to flow through the bypass exhaust pipe 13. At thistime, the flow rate control valve 14 is fully opened to reduce theresistance to the flow of the exhaust gas. In order to enhance theadsorption efficiency the flow rate control valve 14 may be partiallyclosed to reduce the space velocity of the exhaust gas so far as thedischarge of the exhaust gas is not prevented. In this case, control ofthe flow rate control valve 14 is made upon the basis of the amount ofthe exhaust gas which is calculated from the detection result of the airamount detector 16 for achieving optimum control of the flow ratecontrol valve 14. The secondary air pump 9 and the secondary air amountcontrol valve 10 need not be operated. Under this condition, the unburnthydrocarbons in the exhaust gas will be adsorbed by the adsorbent 4.Since all the exhaust gas passes through the main catalytic converter 8aas mentioned above, the temperature of the main catalyst 8 is elevated.

At this time, the temperatures of the adsorbent 4 and the main catalyst8 are monitored based upon the outputs of the exhaust gas temperaturesensor 2, the adsorbent temperature sensor 3, the exhaust gastemperature sensor 5 and the catalyst temperature sensor 7. When thetemperature of the adsorbent 4 comes close to the desorption zone, theexhaust passage change-over valve 31 is brought into a position b andthe flow rate control valve 14 is closed to stop the flow of the exhaustgas through the bypass exhaust pipe 13. On the other hand, the secondaryair pump 9 and the secondary air amount control valve 10 are actuatedcorrespondingly to the operation of the valves 31 and 14 so that freshair is introduced into the exhaust gas flowing through the main passage.Then, the unburnt hydrocarbons in the exhaust gas are burnt with thefresh air prior to reaching the main catalyst 8. Accordingly, theunburnt hydrocarbons will not be externally discharged without beingtreated even if the main catalyst 8 has not reached the light offtemperature at the time when the exhaust gas passage is switched fromthe bypass exhaust pipe 13 to the main passage by means of the exhaustpassage change-over valve 31.

When the main catalyst 8 has reached the light off temperature, theexhaust passage change-over valve 31 is brought into an intermediateposition, i.e. a position between positions a and b to cause the exhaustgas to flow into the bypass exhaust pipe 13 to some extent. Thiselevates the temperature of the adsorbent 4 to the desorption zone todesorb the unburnt adsorbed hydrocarbons. At this time, the flow ratecontrol valve 14 is fully opened to reduce the resistance against theexhaust gas.

Although treatment of the unburnt hydrocarbons per se is possible onlyby the capacity of the main catalyst 8 when the main catalyst 8 hasreached the light off temperature, air is admitted to some extent to theexhaust pipe via the secondary air pump 9 and the secondary air amountcontrol valve 10 in order to prevent the concentration of the oxygenfrom being low and thus being short of oxygen due to the presence of theunburnt hydrocarbons desorbed from the adsorbent 4.

Regeneration of the adsorbent 4 is carried out under the usual runningcondition. In this case, the exhaust passage change-over valve 31 isbrought into an intermediate position as is similar to the case ofdesorption to admit exhaust gas to some extent into the bypass exhaustpipe 13 so that the temperature of the adsorbent 4 is elevated to theregeneration zone. However, it is not necessary to actuate the secondaryair pump 9 and the secondary air amount control valve 10 unlike the caseof desorption. The flow rate control valve 14 is fully opened to reducethe resistance.

If a high load is imposed upon the engine during running, the exhaustpassage change-over valve 31 is controlled and the flow rate controlvalve 14 is closed so that the exhaust gas will not flow into the bypassexhaust pipe 13 since the temperature of the exhaust gas may otherwiseexceed the limit of thermal resistance of the adsorbent 4.

A ninth embodiment will be described with reference to FIG. 10.

The present embodiment is substantially identical with the embodiment ofFIG. 9 except that a converter 20a containing a low temperature activecatalyst 20 is provided between the adsorber 4a and the flow ratecontrol valve 14 in the bypass exhaust pipe 13.

Such an arrangement can prevent the unburnt hydrocarbons from externallydischarging even if the adsorbent 4 should become saturated. The unburnthydrocarbons can be treated with the low temperature active catalyst 20even if the main catalyst 8 has not reached the light off temperaturewhen the adsorbed unburnt hydrocarbons are to be desorbed. Accordingly,it is not necessary to close the exhaust passage change-over valve 31and the flow rate control valve 14 until the main catalyst 8 reaches thelight off temperature. The exhaust gas can be treated more quickly. Thisis advantageous particularly in the case where a very large amount ofunburnt hydrocarbons are emitted due to the temperature of the maincatalyst 8 not being elevated sufficiently which can occur whenrepeatedly starting and stopping the engine in a short period of timesuch as during repair and maintenance.

Temperature control in the above mentioned embodiments will now bedescribed.

The adsorbent 4 should have a sufficient adsorbing capacity. It isdifficult to reduce the size of the adsorber below a predetermined size.It is necessary to increase the surface area of the adsorbent in orderto increase the adsorbing speed. Accordingly, a large space exists inthe adsorbent 4. Therefore, it is hard to make the temperature in theadsorbent uniform throughout the adsorbent. Accordingly, thedistribution of the temperature of the adsorbent 4 is measured by meansof a plurality of thermal sensors.

Control of various valves, etc. is carried out with reference to themaximum temperature in the adsorber 4a. Determination whether thetemperature of the adsorbent 4 reaches the limit of the thermalresistance is of course made with reference to the maximum temperature.Since it is necessary to perform the desorption and regeneration of theentire adsorbent, temperature control is determined with reference tothe minimum temperature for controlling various components.

Detection of temperature in the above mentioned embodiments will bedescribed in detail.

In the foregoing embodiments, the temperature of the adsorbent 4 isdetermined based upon the temperatures in three positions by means ofthe exhaust gas temperature sensor 2, the adsorbent temperature sensor 3and the exhaust gas temperature sensor 5.

Since the temperature is usually higher in the upstream side,determination on adsorption and the limit of thermal resistance is madeby using the detection result of the exhaust gas temperature sensor 2.Since it is considered that the minimum temperature is detected in theexit side, determination on desorption and regeneration is made by usingthe detection result of the exhaust gas temperature sensor 5.

The adsorber temperature sensor 3 is not used for the specific control,but is used for diagnosing whether or not the exhaust gas temperaturesensor 2 and the exhaust gas temperature sensor 5 are functioningnormally. In other words, the results which are detected by the exhaustgas temperature sensor 2, the adsorbent temperature sensor 3 and theexhaust gas temperature sensor 5 are compared with each other. If thetemperatures are distributed as shown in FIG. 11, it is determined thatthe sensors are functioning normally. A slight margin represented in thedrawing as "α" is provided for making the determination of the normalfunction of the sensors.

Control of various valves in each of the above mentioned embodiments ismade based upon control values which are calculated by a given operationof the data from the sensors performed by the control unit 23. Some ofthe inputs and output to and from the control unit 23 are shown in FIG.12.

In accordance with the present invention, the unburnt hydrocarbons arenot externally discharged without being treated even when an exhaust gastreating catalyst does not function, for example, immediately afterstarting of an engine mentioned above. Although embodiments in which theunburnt hydrocarbons are adsorbed have been described, it is to beunderstood that the present invention is also applicable to theelimination of NOx and other harmful materials.

Although the invention has been described and illustrated in detail, itis to be clearly understood that the same is by way of illustration andexample, and is not to be taken by way of limitation. The spirit andscope of the present invention are to be limited only by the terms ofthe appended claims.

What is claimed is:
 1. A system for purifying an exhaust gas for use inan automobile, comprising:an exhaust gas main passage for dischargingthe exhaust gas from an engine; a bypass passage for connecting anupstream portion of said exhaust gas main passage with a downstreamportion of the exhaust gas main passage; adsorbent disposed in saidbypass passage having a temperature range at which said adsorbentadsorbs unburnt hydrocarbons, (the adsorption zone), a temperature rangeabove the adsorption zone, at which said absorbent desorbs the adsorbedunburnt hydrocarbons therefrom, (the desorption zone), and a furtherhigher temperature range at which the adsorbed materials which can notbe completely desorbed in said desorption zone can be eliminatedtherefrom (the regeneration zone; exhaust gas treating catalyst providedin said exhaust gas main passage in parallel with said bypass passage;means for detecting the temperature of at least one of said adsorbentand said catalyst; air supplying means disposed in said bypass passageupstream of said adsorbent; passage changing means provided at the exitof said bypass passage for adjusting the ratio of the flow rate of theexhaust gas through said exhaust gas main passage to that through saidbypass passage; exhaust gas recirculating means which connects saidbypass passage downstream of said adsorbent with an air intake system ofsaid engine for returning a desired amount of the exhaust gas to saidair intake system; means for controlling said passage changing means insuch a manner that the exhaust gas is caused to flow through said bypasspassage when the temperature of said adsorbent falls into the adsorptionzone and said bypass passage is closed to cause the exhaust gas to flowthrough the main passage and air is supplied to said bypass passage bysaid air supply means when the temperature of said adsorbent falls intothe desorption zone.
 2. A system for purifying an exhaust gas for use inan automobile as defined in claim 1 and further including subsidiarybypass means for introducing the exhaust gas into said bypass passage inthe upstream side of said adsorbent from said exhaust gas mainpassage;said control means being capable of elevating the temperature ofsaid adsorbent up to the regeneration zone by causing the exhaust gas toflow into said bypass passage via said subsidiary bypass passage.
 3. Asystem for purifying an exhaust gas for use in an automobile as definedin claim 2 in which means for detecting the amount of air supplied tosaid engine is provided;said control means being capable of controllingthe amount of the exhaust gas which is recirculated by said exhaust gasrecirculating means based upon the detection result of said air amountdetecting means.